Physical activity (PA) is fundamental to human life. With comfort and easiness as the goal, automation and modernization in the past 50 years has dramatically reduced our PA, and completely changed our life style and work environment. Sudden decrease in physical activity can lead to body energy imbalance and diseases such as obesity and cardiovascular dysfunction, as evidenced by NHLBI recently announcement: "About 97 million adults in the United States are overweight or obese. Obesity and overweight substantially increase the risk of morbidity from hypertension; dyslipidemia; type 2 diabetes; coronary heart disease; stroke; gallbladder disease; osteoarthritis; sleep apnea and respiratory problems; and endometrial, breast, prostate, and colon cancers." The government health website (http:/Iwww.health.govlhealthypeop(e/LHI/) lists PA as the leading health indicator for the Nation over the next 10 years. Considering that 10% of positive energy imbalance can lead to 15 lbs of weight increase per year, an accurate device for quantifying PA and energy expenditure (EE) in free-living conditions becomes critical. This proposal plans to improve and validate Intelligent Device for Energy Expenditure and Activity (IDEEA), developed by MiniSun LLC. for measuring PA and EE in free-living humans. The prototype demonstrated dramatic improvement and technical breakthroughs in functions and accuracy over previously available devices. The device records body posture and movement for 24-hrs or longer with time accuracy to sub-second, identifies 21 types of postures, 17 types of limb movement and posture transitions, and 6 types of major PA, including sleeping, reclining, sitting, standing, walking, ascending stairs, descending stairs, running, and jumping with 98.5% accuracy. IDEEA also computes the distance and speed of locomotion, the mechanical work and power of PA, and estimated EE of these activities using the state-of-art methods and techniques. We plan to improve the hardware/software design for ease of use by researchers and to reduce the production cost. We also plan clinical studies to further validate the device.